Neil Mcormick of the Telegraph has a decent theory on why it is people tend to believe that music is never as good as it once was; best encapsulated in the phrase: "it's all shit these days."
"I think the answer lies in the way we listen to music and in particular the level of intensity and concentration we bring to something that, I would guess, most adults consider a pleasurable diversion rather than the core of their very being. Pop is neophyte by instinct. It occupies such a central place in modern youth culture that it is almost compelled to keep changing, embracing ever more sonic, rhythmic and lyrical extremes so that each successive generation can claim a badge of musical identity that is their own. Given its relative importance in their social and cultural life, young people (like music critics) tend to be willing to put in the time and summon the genuine effort sometimes required to crack musical codes and find the key that unlocks each new genre. And, of course, you have to be prepared to sift through a great deal of dross to get to the gold. It was ever thus, but as people get older and, by choice or necessity, dedicate less energy to the listening process, the inevitable tendency is to stick to what you know and respond to things that sound familiar."